The present disclosure relates to a resistance variable memory device that writes data by using a spin transfer effect based on injection current.
Data communication apparatuses, in particular, personal small-sized apparatuses such as portable communication terms have widely and increasingly been used among consumers. Along with this popularity, enhanced performance, such as higher integration, increased processing speed, and lower consumption of power have been demanded for devices employed in such apparatuses, such as memory and logic devices. In particular, nonvolatile memories are considered as significant components for enhancement of the performance in such apparatuses.
The nonvolatile memories practically used in the related arts include semiconductor flash memories and ferroelectric nonvolatile memories (FeRAMs). These memories have been actively researched and developed for achieving higher performance.
Lately, as a nonvolatile memory using a magnetic material, magnetic random access memories (MRAMs) utilizing a tunnel magneto-resistance effect have been disclosed and attracted much attention (for example, refer to “J. Nahas et al., IEEE/ISSCC 2004 Visulas Supplement, page 22”).
The principle of operation of an MRAM will be described briefly.
The MRAM is a magnetic data recording device having a structure in which minute storage carriers made of magnetic materials are regularly arranged and wirings are wired so as to allow accessing the respective storage carriers.
When current is allowed to flow into both conductive lines (word lines) and readout conductive lines (bit lines) which are disposed above or below the magnetic storage carriers, a combined current magnetic field is generated. Data are written to the MRAM by controlling the magnetization of each magnetic material by using the combined current magnetic field.
Generally, depending on the direction of the magnetization, data of “0” and “1” are stored. As a typical method of rewriting data of a device, a method that utilizes asteroid characteristics is known (for example, refer to Japanese Unexamined Patent Application Publication No. 10-116490). Moreover, a method that utilizes switching characteristics (for example, refer to US Patent Publication No. 2003-072174).
The data are read out as follows. A memory cell is selected using an element such as a transistor and the direction of magnetization is extracted as a voltage signal in accordance with a current magnetic effect.
As a proposed film structure of the memory cell, there is known a three-layer junction structure (ferromagnetic tunnel junction; magnetic tunnel junction, abbreviated to MTJ) including a ferromagnetic material, an insulating material, and a ferromagnetic material. This structure will be referred to as an MTJ structure.
In the MTJ structure, one ferromagnetic layer is used as a fixed reference layer in which the magnetization direction is fixed, and the other ferromagnetic layer is used as a recording layer (free layer). In this way, the MTJ structure causes the direction of magnetization of the recording layer to correspond to the voltage signal in accordance with the tunnel magneto-resistance effect.
The MRAM is capable of rewriting the data “0” and “1” by reversing the magnetization directions of the magnetic materials at high speed and substantially without a limit of times (1015 or more). This is the major advantage of the MRAM as compared with other nonvolatile memories.
On the other hand, the MRAM consumes much power since a large amount of current (for example, several mA to several tens of mA) has to be passed to the wirings. Moreover, since the MRAM has to include both the word lines for recording and the bit lines for reading, it may be difficult to reduce the size of the memory cells. Furthermore, when the size of the MTJ structure is decreased, the magnetic field necessary for the magnetization reversal increases. Thus, the MRAM is disadvantageous in scaling from the power consumption perspective.
As one solution to the problems, recording methods that do not use the current magnetic field have been researched. Among them, a recording method that uses magnetization reversal based on spin-transfer has been actively researched (for example, refer to U.S. Pat. No. 5,695,864).
A memory device employing the spin-transfer magnetization reversal is configured by the same MTJ structure as the MRAM. The spin-transfer magnetization reversal utilizes the fact that when spin-polarized electrons passing through a magnetic layer where the magnetization direction is fixed enter a free layer, a torque is applied to the magnetic layer. Specifically, when current of a threshold value or more flows, the magnetization direction of the free layer is reversed.
The data “0” and “1” are rewritten when the polarity of the current is changed.
The absolute value of the current for achieving the magnetization reversal is several mA or less in a memory device of about 0.1 μm scale, and the absolute value decreases in proportion to the volume of the memory device. In this respect, the memory device employing the spin-transfer magnetization reversal is advantageous in scaling.
Moreover, since the memory device employing the spin-transfer magnetization reversal is not necessary to include the word lines for recording, which are necessary for the MRAM, there is such an advantage that the configuration of memory cells can be simplified.
The data are read by utilizing the tunnel magneto-resistance effect similar to the MRAM.
In this specification, the MRAM that utilizes the spin-transfer will be referred to as a spin transfer random access memory (SpRAM). In addition, a spin-polarized current for causing the spin-transfer will be referred to as a spin injection current.
Great expectations are focused on the SpRAM as a nonvolatile memory enabling to realize low power consumption and large storage capacity while maintaining the advantages of the MRAM which is capable of rewriting data at high speed and substantially without a limit of times.
In the proposed SpRAM, the data “0” and “1” are rewritten by changing the polarity of the spin injection current.
However, the result of the magnetization reversal may not necessarily be determined by only the polarity of the spin injection current due to the natural instability of the spin-transfer magnetization reversal.
In the SpRAM, in addition to the magnetization state corresponding to the data “0” and “1,” a quasi-stable state exists which is realized only when the spin injection current flows. The instable result of the magnetization reversal results from a phenomenon in which the magnetization state being presently trapped in the quasi-stable state becomes indefinite after the current stops flowing.
It is therefore desirable to provide a resistance variable memory device which can be driven by a spin injection current, thus achieving a stable magnetization reversal over a wide range of injection current.